Two closely series of poly(ester imide)s had been synthesized by solution polycondensation of p-phenylenebis(trimellitate) dianhydride with aliphatic diamines. The differential scanning calorimetry (DSC) traces of the most poly(ester imide)s exhibited two endotherms representing the solid state to anisotropic phase transition (T-m1) and the anisotropic to isotropic melt transition (T-m2), respectively. Observation under polarizing microscope and wide-angle X-ray diffraction (WAXD) measurements suggested that the anisotropic phase formed above the melting paints (T-m1) had a smectic character. The thermogravimetric analyses (TGA) revealed that the thermal stabilities of the poly(ester imide)s were up to 350 degrees C.